Electroacoustic translating device



Sept. 11, 1934.

A. R. BARFIELD 1,973,277 ELECTROACOUSTIC TRANSLATING DEVICE Filed Aug. 23, 1933 2 Sheets-Sheet 1 FIG. :1.

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INVENTOR ATTORN EY Sept. 11, 1934. R BARHELD i I 1,973,277

ELECTROACOUST I C TRANSLAT ING DEVICE Filed Aug. 23, 1933 2 Sheets-Sheet 2 FIG. 5 m as 10 M as INVENTOR flm/rew IE. Ba 1772 H Patented Sept. 11, 1934 UNITED STATES ELECTROACOUSTIC TRANSLATING DEVICE Andrew R. Barfield, East Orange, N. J., assignor, by mesne assignments, to George G. Cromartie and Einer W. Nielsen, both of Maplewood, N. J.

Application August 23, 1933, Serial No. 686,362

32 Claims.

This invention relates to electro-acoustic translating devices and more particularly to devices of the electro-magnetic type, adapted to be used as loud speakers in radio receiving apparatus, electric phonographs and the like.

It is an object of this invention to provide a device of the class described'which shall. have relatively high sensitivity and an improved performance characteristic, particularly in respect to the volume of sound output, the range or band of audible frequencies to be covered, and also in respect of the elimination or reduction of certain types of distortion heretofore usual in such apparatus.

It is an object of my invention to provide a device of the class described which is relatively insensitive to shock excitation such as caused by bursts of static or interference such as closure and opening of electric switches in the neighborhood and the like, without sacrificing sensi tivity on desired sounds and musical tones.

It is a further object of my invention to provide apparatus of the class described having relatively simple adjustments for controlling the pitch or resonance of the device, if desired, and for adjusting the position and stiffness of the armature system.

It is a further object of my invention to provide apparatus of the class described which is well adapted for quantity production by relatively unskilled labor and in which the various parts are so designed and correlated that even when produced in quantities by relatively unskilled labor the various individual devices will have a high level of'consistency.

It is a further object of my invention to provide a loud speaker which shall have high sensitivity and an improved performance characteristic in respect of decreased distortion within said frequency band, and in general to produce a simple and highly efficient device of the class described.

Still other objects and advantages of my invention will be apparent from the specification.

The features of novelty which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its fundamental principles and as to its particular embodiments, will best be understood by reference to the specification and accompanying drawings, in which Fig. 1 is a rear elevation view partly in section of an electro-acoustic translator in accordance with my invention;

Fig. 2 is a side elevation view thereof partly in section, and

Fig. 3 is a perspective view of the armature or vibrating system.

Fig. 4 is a top plan view of one of the plates 1 of the motor framework.

Fig. 5 is a side elevation view thereof,

Fig. 6 is a plan view of the pole piece laminations, and

Fig. 7 is a diagrammatic view of a modified 5 form of mechanical transformer.

While in the preferred embodiment illustrated in the drawings I have shown a sound reproducer of the so-called direct-acting type, that is, one which is adapted to operate without the use of amplifying horns, resonance boxes or the like, it will be apparent that my invention is not limited thereto and may be applied in various 'ways with such devices, and it will also be understood that the apparatus is by its nature reversible and may be used as microphone or telephone transmitter if desired.

Referring now more particularly to the drawings, 1 designates the frame or foundation ring of the instrument, which may be in the form of an annular rim the outer edge of which is turned outward to form a shoulder. The rim 1 may be provided with a plurality of inwardly oifset brackets 2 herein shown as four in number, to which the operating mechanism may be secured. s5

Positioned within the rim 1 there may be provided an acoustic diaphragm of any desired shape and form adapted to be driven by the motor mechanism hereinafter to be described, or to drive the said motor mechanism.

In what is for certain purposes the preferred embodiment of my invention, the diaphragm may be in the form of a cone 3 provided with concentric annular rings 4 adjacent its periphery, and the outer edge of the rim is secured in any suitable way to the rim 1; for example, annular rings 5 of cardboard, felt or the like may be glued or otherwise secured to opposite sides of the rim of the diaphragm and within the annular ring 1.

The diaphragm itself may be of any suitable material, such as paper, fabric, or the like and preferably is made somewhat thinner at or adjacent the corrugations 4 to impart additional flexibility to the diaphragm, and it will be understood that types of diaphragms other than that shown may be utilized if desired.

Positioned at a suitable point on the diaphragm, such for instance as at the apex of the cone here shown, I provide a cap member 6 which may be in the form of a metallic cone having attached to it a rod 7, the rod 7 being in turn'attached to the end of the armature assembly designated generically as 8.

The armature assembly may be provided with a mounting spring 9 which is preferably rigidly secured to the motor frame work and permits, by reason of its torsional flexibility, the armature assembly to rock about the spring 9 as a center.

The armature itself preferably consists of a multiplicity of lamination gathered together in a block 10 of rectangular shape and is suspended in the magnetic field produced, for example, by magnets 11 and 12. These magnets preferably are of horse-shoe shape and are positioned on the motor frame work with their legs defining a plane normal to the annular rim 1, with their like poles adjacent each other and on opposite sides of the armature assembly, whereby the fiux path is in a direction parallel to the rod '7, or perpendicular to the face formed by the edges of the armature laminations.

In its preferred form the motor assembly itself may comprise a pair of spacer plates 13 and 14 of suitable nonmagnetic metal such as brass, of generally oblong shape but having their ends rounded 01f to conform to the shape of the magnets. Each of the plates 13 and 14 is preferably provided with a central rectangular opening 13a adapted to receive the armature assembly, and with circular locating depression 130 at opposite ends to receive locating lugs, as will be hereinafter described, and also with long and relatively shallow notches 131; on opposite sides to receive the laminations to form the pole pieces. Also threaded openings 13d may be provided to receive bolts to secure the motor assembly in position and lugs 13; may be provided to support the 0011.

The dimensions which must be accurately controlled are (l) the distance (d) from shoulder to shoulder on plates 13 and 14, (2) the distance (e) on the pole piece laminations, and (3) the thickness (t) of the armature; when these are made exactly as calculated, the parts fit together without adjustment or shimming.

For the purpose of providing a wider surface to engage the pole piece laminations, tapered shoulders 13c may be provided extending across the plates 13 and 14. When the plates 13 and 14 are so formed they are duplicates and the motor assembly may be formed from two of them, the faces carrying the projections being merely turned toward each other, thus reducing the cost by avoiding the necessity for separate-dies for the two plates.

The armature assembly, as already described, comprises the armature proper 10 made up of a multiplicity of laminations of magnetic material and the fixed armature supports 15 and 16. In the form shown these are preferably posts, the opposite ends of which are drilled and tapped to receive bolts.

The armature spring 9 extends longitudinally through the armature 10 through suitable openings provided in the laminations and into openings in posts 15 and 16 at its opposite ends and is soldered or otherwise suitably secured in position, so that both the posts 15 and 16 and the armature assembly 10 are all rigidly secured upon the spring 9. When posts 15 and 16 are firmly secured in the motor assembly, this construction permits a rocking motion of the armature assembly 8 about the spring 9 as an axis by reason of the flexibility of the spring 9 in the space between the posts 15 and 16 and the armature assembly 10.

In the form shown'the spring 9 is rectangular and flat in cross-section, but it will be understood that other forms of springs may be utilized, if desired, and I prefer to provide notches in the spring 9 between the posts 15 and 16 and the armature per se in order to increase the torsional flexibility of spring 9 and still provide a large contact surface between the spring, 9, and the parts to which it is secured.

A lever arm 8a is rigidly secured to the armature and is preferably of a suitable form to impart extreme rigidity with minimum weight, and for this purpose I find it advantageous to utilize a frame of light, strong material such, for example as duralumin, or the like, consisting essentially of a three-sided member having a rear face 19 and side members 1'7 and 18.

The rear face 19 preferably tapers from substantially the full width of the armature at the base to almost a point at the top and the side members likewise taper in width from substantially the full width of the armature at the base to a small width at the top, and the rear face may be provided with openings 20 and 21 to economize in weight and to prevent parasitic vibrations of a large flat surface. The side faces 17 and 18 preferably are continued to the base of the armature in.the form of wings or arms 22 and 23 which embrace thelamination plates, and are given a slight bend so as to be parallel thereto. In addition, reinforcing plates 25 and 26 shaped to conform to the side plate may be suitably secured on the outside of the side plates at the base of the armature.

In the form shown the reinforcing plates 25 and 26 may be riveted to the side members by means of rivets 27 at their forward end and additional rivets 27a passing through the lamina tions may be provided. Thus it will be seen that the armature assembly is relatively extremely rigid and at the same time the apparent weight is relatively light so when vibrational forces are 1115 applied at point 28, Fig. 3, the mass reaction of the assembly is small.

The actuating winding or so-called voice coil 75 may be wound upon a suitable form having an opening in the center adapted to surround the armature assembly, as will be seen from Figs.

1 and 2. I

In building up the motor, the armature assembly with the voice coil surrounding it is placed between upper and lower plates 13 and 14 and bolts 35 are passed through holes 13:1 in both the upper and lower plates and into the posts 15 and 16.

The lamination members '70 which form the pole pieces are preferably given the general shape of the Greek letter 1r as shown in Fig. 6, in which the ends 60 and 61 form the pole faces cooperating with the armature, and the face 63 engages the inner face of the magnets.

For convenience in stacking the laminations, 13,: one of the ends is rounded off at 62 so that all of the lamination may be easily faced the same way. This causes the concavities made by the stamping die to lie on the same side.

To continue the motor assembly, a group of 146 the lamination pieces corresponding in width to the width of the depression 13b is inserted in the said depression with the members 60 and 61 on opposite sides of the voice coil. To secure the laminations in position the magnets, preferably 1. .5 in unmagnetized condition, are placed in the position indicated by dotted lines in Fig. 4.

For this purpose the magnets are made preferably slightly narrower than the width of plates 13 and 14 and a tool is applied which slightly 1E0 separates the opposite faces. The magnets are thus dropped in position around plates 13 and 14, the tool being withdrawn during the process. This permits the magnet to contract and tightly grip the laminations in the opposite notches 13b, thereby serving the double purpose of securing the laminations and magnets tightly in position, and maintaining the minimum magnetic reluctance in the magnetic circuit, due to the clamping engagement between the inner pole faces of the magnet and the outer edges of the laminations. The magnets are preferably placed in position before being magnetized, and the magnetizing operation performed subsequently.

This construction of the magnetic circuit permits the use of an air gap which is very much shorter than in conventional practice and in my preferred embodiment it may be of the order of 0.003 to 0.004", whereas in ordinary practice it is of the order of 0.008".

This results in a number of'advantages. The desired flux density may be produced by a relatively smaller magnetic system, thereby reducing the cost of the magnets. It also provides economy of space since the same magnetic flux density can be produced with smaller magnets than heretofore used.

It also provides a better high frequency response in the device, since the impedance over the band of audible frequencies is more nearly uniform with the short air gap; that is to say, the ratio is a minimum, where L is the inductance of the coil as placed in the system and Z is its total impedance including motional impedance.

A still further advantage of this construction is that it results in a considerably higher obtainable efficiency of the device, or, as a loud speaker a greater volume of sound output for a given input.

It should be noted that the reduction of the air gap made possible by this construction is of such magnitude that, whereas in prior practice it was usual to neglect the reluctance of the magnetic path through the iron (because of the fact that with a large air gap the reluctance is essentially concentrated in the air gap) in the present construction the reluctance of the air gap is relatively so much decreased that in calculating the design the reluctance of the path in the iron can no longer be neglected and a laminated magnetic path is used.

For the purpose of securing the motor to the speaker assembly I have provided the cruciform bracket having the four arms designated as-30, the upper two arms terminating in contact plates 31 at the outer end; and the lower two arms having contact faces 32 all adapted to engage brackets 2 and the lower arms also having flat base faces 33 adapted to engage the upper plate 13 of the motor.

The lower surfaces of the lower legs 30 may be flat as indicated by 33 and of sufficient width to engage the opposite legs of the magnets, and are preferably provided with cylindrical, downwardly extending lugs 34, adapted to engage in the depressions 130 in the upper plate 13, thereby properly locating the motor with reference to the frame. If desired a lower cover plate may be provided and the entire assembly may be secured by means of bolts 66 and 67 passing through the cover plate and en aging in members 33.

For the purpose of centering the armature and of controlling the stiffness and thereby the fredecreasing the free length of spring arm 47, and,

quency response of the apparatus, I provide a spring, the natural period and neutral position of which is adjustable, and which is connected to the vibrating system. For this purpose the center of the cruciform bracket may be provided with a flat portion 37 having a transverse fulcrum 38 preferably in the form of a knife edge. A pair of blocks 39 and 40 are adapted to be mounted upon the knife edge 38 and to be secured thereon by bolts 43 and 44 passing through elongated clearance slots 41 and 42.

It will be understood that by adjusting bolts 43 and 44 the blocks 39 and 40 may be rocked upon the fulcrum 38 in the fashion of a seesaw. Clamped between the blocks 39 and 40 there is preferably provided spring 47 and for this purpose depressions may be provided in the inner faces of blocks 39 and 40 to accommodate the spring and for the purpose of facilitating assembly each of the blocks 39 and 40 may be provided at one side with a cylindrical stud 45 while the other is provided with a cylindrical depression 46.

When the blocks are assembled with the spring between them the projections and depressions engage each other to make a proper fit. The spring 47 may be provided at its upper end exteriorly of the block with a transverse slot 48 and the block 40 may be provided with a tapped opening to receive set screw 49 carrying a collar 50 which engages in the slot 48.

It will thus be seen that by loosening screws 43 and 44 and by rotating set screw 49 the blocks 39 and 40 are caused to move upwardly or downwardly, sliding over the knife edge 38 and thereby as will be understood, changing its stiffness and the natural period of the moving system.

For the purpose of increasing the stiffness of the outer portion of spring arm 47 and concentrating its flexibility in substantially the portion adjacent the blocks 39 and 40, the outer edges may be bent inwardly as indicated to form flanges 51 and the lower portion may be slotted as at 52, to receive the pin 53 which is preferably secured at one end to the lower end of the spring arm 47 and at its opposite end to the point 68 on the armature assembly.

The upper end of the armature assembly may be connected at point 28 with the driving rod 7 which goes to the apex of the diaphragm.

The operation of the apparatus will be clear from the foregoing. When voice currents are supplied to the voice coil 75, as will be understood, one end of the armature assembly tends to become north while the other end tends to become south. This in turn introduces a rotationalcouple tending to rotate the armature about the spring 9 in one direction. If the current is reversed the direction of rotation tends to become the opposite. 135

It will be understood that this motion of the armature assembly is communicated through the driving rod '7 to the diaphragm, causing it to vibrate in a manner corresponding to the currents impressed on the voice coil, and, conversely, if the diaphragm is set into vibration by means of sound waves falling upon it, its motion is transmitted to the armature assembly and correspondingly generates currents in the voice coil.

By adjustment of the stiffness and position of the spring 47 as above mentioned, the desired stifiness, centering and restoring force may be applied to the armature assembly.

The mechanical construction of the preferred form of my invention having been described in 15 It will be understood that the motional im-- pedance. of the diaphragm is dependent, among other factors, upon its size, and it will also be understood that the amplitude through which the diaphragm should be driven to produce a given sound output willin general be the greater,

the smaller the diaphragm.

-While my invention is not limited in its application to a diaphragm of aparticular size, it is particularly welladapted to relatively small diaphragms. For-example, excellent results have been obtained with a diaphragm the outside diameterof which is 5".

In the construction indicated, if the diaphragm has this diameter and the maximum amplitude of motion of the armature is utilized in an air gap of not more than 0.004", a relatively high step-up ratio between the motion in the air gap and the motion applied to the apex of the diaphragm is desirable, and I have found that excellent results have been obtained using a step-up ratio of about -1 to 13.

With a diaphagm and air gap of such size and a step-up ration of such an order it is found that the proper mechanical load is applied to the motor to obtain maximum efliciency, while at the same time the amplitude of motion of the diaphragm necessary particularly for low frequency response is obtained very satisfactorily.

I have found'that it is advantageous to obtain the desired step-up ratio from a mechanical transformer which is incorporated as a physical part of the armature assembly, rather than to apply the vibration of the armature to a physically separate step-up ratio arm.

The reason for this is that the reaction of the diaphragm applies to the point 28 a force equal to the force applied to move the diaphragm. Since the force developed by the lever arm is inversely proportional to the length of the arm, and since action and reaction are always equal, the reaction of the diaphragm on the lever arm applied at point 28 also inversely proportional to the length of the lever arm, or the greater the step-up ratio, other things being equal, the less is the force applied to the lever arm by the diaphragm. In order for the system to be in equilibrium. another force must be applied to the lever arm which is equal in magnitude and opposite in direction to the force applied to the lever'arm by the reaction of the diaphragm and this force is provided by a transverse deflection oi. the annature spring 9. Consequently the less the step-up ratio the greater must be the resistance of armature spring 9 to transverse deflection, and this has heretofore required a spring so stiff thatsubstantial undesired high frequency resonance has been introduced. By increasing the step-up ratio the force tending to deflect the armature is correspondingly reduced, which permits the use of a spring less resistant to transverse deflection.

If the step-up ratio were infinite, the deflecting force tending to bow spring 9 would be zero, and the spring could then be calculated for torsional stifiness alone. With step-up ratios of the order here utilized, I find that the spring 9 can be calculated for torsional stiffness alone, provided the armature assembly is so designed that its center of percussion with respect to the point of attachment to the load coincides with the axis of vibration of the armature.

In this connection, it may be noted that it may in some cases be desirable to utilize a stepup ratio in excess of that desired, and to step back down to the desired ratio through a second mechanical transformer, by providing a. longer lever arm on the armature, and a second lever arm, as shown in Fig. 7,-in which the point 28 is connected to the free end of a step-down lever 71, fixed at point '72, and connected as at 73 to the diaphragm. This construction would be useful in the case of cones somewhat larger than herein considered and where exceptionally good quality is desired.

If the armature assembly has its mass so distributed that the center of percussion with respect to the point of the cone attachment coin cides with the position of the armature spring 9, then any translatory motion of the armature resulting from transverse deflection of spring 9 already described will not produce translational motion of the rod '7, but tends only to rotate the armature as a whole about the point of attachment to rod 7, as the center of rotation, it being understood that the axis of suspension of the armature or axis of vibration and the center of percussion in respect thereto are interchangeable; that is, that the axis of vibration of the armature, i. e., the spring axis, may be considered either as the axis of suspension or the center of percussion and that the point of attachment of the cone may be regarded either as the center of percussion or as the axis of suspension respectively. This I have found reduces very materially the undesired noises produced by conventional types of magnetic speakers.

In satisfying this condition application is made of the equation where l is the distance from the armature spring 9 to the point of attachment to the cone, K is the radius of gyration of the movable armature assembly about the armature spring 9, and d is the distance from the spring 9 to the center of gravity of the armature assembly. If I is fixed by the step-up ratio desired, then the equation may be satisfied by changing the position of the center of gravity of the armature system, in a number of ways, for instance. by attaching weights unsymmetrically on the armature, by using laminations which are mounted oif center, or by cutting out portions of the laminations off center. It will be noted that the point 28 is slightly offset from the plane of spring 9. I have found that as a practical matter, this does not interfere with the design of the armature so that the center of percussion with respect to point 28 coincides with spring 9. However, if desired the upper end of the lever arm may be dished or offset inwardly so that point 28 lies in the plane of spring 9, as will be understood.

By the described construction, resonances in the high frequency band are considerably reduced in their effect as compared to their effect in other types of magnetic speakers. This results in a speaker which is less noisy in operation, particularly when static is present, because a large part of the noise radiated under such condition is due to the shock excitation setting up a high frequency vibration at one or more of the natural periods of the system. By reducing the .high frequency resonance, the noise so radiated modifications and changes may be made without departing from the spirit and scope of my invention, as will be understood by those skilled in the art.

I claim: 1. In apparatus of the class described, in com bination, an armature assembly supported for oscillatory vibration, magnetic pole pieces on .opposite sides of opposite ends of said armature,

and a spring supported independently of the support of said armature secured at its vibratile end to said armature assembly, for applying a restoring force thereto,said spring being adjustably mounted in a manner to permit adjustment of the natural frequency of the vibrating system.

2. In apparatus of the class described, in combination, an armature, means for supporting said armature for oscillatory vibration, magnetic pole pieces on opposite sides of opposite ends of said armature, and adjustable means secured to the armature-assembly independently of the support of said armature for controlling the vibration of said armature, said means comprising a spring longitudinally adjustable to vary the natural frequency of the vibration system.

3. In apparatus of the class described, in combination, an armature, a lever arm embracing and attached to said armature, means for supporting said armature for oscillatory vibration, a vibratory spring and a mechanical connection between said spring and said lever arm.

4. In apparatus of the class described in combination, an armature, a lever arm embracing and rigidly secured to said armature, means for supporting said armature for oscillatory vibration, a vibratory spring mechanically connected to said lever arm, and means for adjusting the natural period of said spring.

5. In apparatus of the class described, in combination, an armature, a lever arm embracing and rigidly secured to said armature, means for supporting said armature for oscillatory vibration, a vibratory spring connected to said lever arm, and means for adjusting the neutral position of the vibratile end of said spring. I

6. In apparatus of the class described, in combination, an armature, a lever arm embracing and rigidly secured to said armature, means for supporting said armature for oscillatory vibration, a vibratory spring connected to said lever arm, and separate means for adjusting the neutral position of the vibratile end of said spring and the natural period thereof.

7. In apparatus of the class described, in combination, an armature, a lever arm rigidly. secured to said armature, means for supporting said armature for oscillatory vibration, magnetic pole pieces at both sides of opposite ends of said armature, a vibratory spring having its vibratile end secured to said lever arm, a support for the fixed end of said spring, comprising a fulcrum and a spring engaging member mounted for rocking adjustment on said fulcrum and means for varying the length of the vibratile portion of said spring.

8. In apparatus of the class described in combination, a pair of spacer plates of non-magnetic material, having notches on opposite sides in the outer edges thereof, the inner edges of said notches forming lamination locating shoulders, lamination plates of magnetic material positioned in said notches against said shoulders and extending toward each other, said lamination plates having their intermediate portions cut away to define a coil receiving space, a coil positioned in said space, a vibratory armature positioned within said coil, a source of magnetomotive force embracing said lamination plates and abutting the back edges of all said lamination plates.

9. In apparatus of the class described, in combination, a pair of spacer plates of non-magnetic material, having notches on opposite sides in the outer edges thereof, the inner edges of said notches forming lamination locating shoulders, laminated plates of magnetic material positioned in said notches against said shoulders extending between said spaced plates and toward each other, and having their intermediate portion cut away to define opposite pole pieces and an intermediate coil receiving space, a vibratory armature having supporting posts torsionally secured thereto, means for securing said posts between said spaced plates, and a source of magnetomotive force embracing said lamination plates abutting the back edges thereof.

10. In apparatus of the class described in combination, a pair of spacer plates of non-magnetic material having notches on opposite sides in the outer edges thereof, the inner edges of said notches forming lamination locating shoulders, laminated plates of magnetic material positioned in said notches against said shoulders extending between said spaced plates and toward each other, and having their intermediate portion cut away to define opposite pole pieces and an intermediate coil receiving space, a vibratory armature having supporting posts torsionally secured thereto, means for securing said posts between said spaced plates, a pair of magnets closely embracing the exterior edges of said laminated plates and abutting the back edges thereof.

11. In apparatus of the class described, an armature assembly comprising, in combination, a laminated armature of magnetic material, a lever arm partly enclosing said armature and rigidly secured thereto, and a torsion spring extending transversely through said armature, and having its ends projecting beyond said laminations.

12. In apparatus of the class described, an armature assembly comprising, in combination, a stack of laminations forming an armature, a lever arm comprising wings engaging opposite ends of said stack of laminations, means extending through said wings and said stack of laminations for securing the same together, and means for mounting said armature for torsional vibration about an axis passing longitudinally through said stack of laminations.

13. In apparatus of the class described, an armature assembly comprising, in combination, a stack of laminations forming an armature, a lever arm comprising oppositely extending portions embracing said stack of laminations between them, said lever arm extending out beyond the end of said'laminations on one side thereof, and means for rigidly securing said 1ever arm and said laminations together.

14. In apparatus of the class described, an armature assembly comprising, in combination, a stack of laminations.:forming an armature, a

' tapered lever arm having wings at its base enver arm partly enclosing said armature and rigidly secured thereto, and a torsion spring passing through said laminations and having its end portions projecting beyond said armature at opposite ends and adapted to be secured to a support to permit vibration of the armature assembly.

16. In apparatus of the class described, an armature assembly comprising, in combination, a laminated armature of magnetic material, a lever arm having portions lying alongside and embracing said armature and rigidly secured thereto, a torsion spring extending transversely through said armature, and mounting posts secured to opposite ends of said spring and spaced from said armature.

1'7. In apparatus of the class described, in combination, a diaphragm, a vibratory armature for vibration with the said diaphragm, magnetic pole pieces on opposite sides of opposite ends of said armature, the motional impedance of said diaphragm being relatively less than that required to properly load said armature, and a unitary mechanical transformer for matching the impedances of said diaphragm and armature, said transformer being a lever arm at least partly surrounding and embracing said armature and rigidly secured to said armature as a part of the armature assembly, and having its free end secured to said diaphragm.

18. In apparatus of the class described, in combination, a diaphragm, a vibratory armature for connection to said diaphragm, magnetic pole pieces on opposite sides of opposite ends of said armature, the "motional impedance of said diaphragm being small in comparison with that required to properly load said armature, and a single unitary mechanical transformer for matching the impedances of said armature and diaphragm, said transformer being an arm embracing said armature on opposite sides thereof and rigidly secured thereto for vibration therewith about the same axis, and having its free end secured to the said diaphragm.

19. In apparatus of the class described in combination, a vibratory armature assembly comprising an armature of magnetic material, means for supporting said armature for vibration about an axis therein, a lever arm rigidly secured thereto for vibration therewith and adapted to be connected at its free end to an acoustic element, the mass of said armature and lever arm being so distributed that the center of percussion thereof with respect to the point of attachment to said acoustic element coincides with the axis of vibration of said armature.

20. In apparatus of the class described in combination, a diaphragm, a vibratory armature for connection to said diaphragm, the motional impedance of said diaphragm being less than that required to properly load said armature, and a pair of mechanical transformers interposed between said armature and said diaphragm, the first being arranged to provide a substantial motion step-up ratio in excess of that desired and in excess of 1:2, and the second to provide a motion step-down ratio.

21. In apparatus of the class described, in combination, a vibrating armature assembly comprising an armature of magnetic material, means for supporting said armature for vibration about an axis thereon, a lever arm rigidly secured thereto for vibration on the same axis and adapted to be connected at its free end to an acoustic vibrator, the constants of the armature assembly being so chosen that v where l is the distance between the axis of vibration of the armature to the point of attachment of the acousticvibrator, K is the radius of gyration of the armature assembly about the axis of vibration, and d is the distance from the axis of vibration to the center of gravity of the armature assembly. H i

22. In apparatus of the class described, in com bination, an armature, a lever arm rigidly secured to said armature for vibration therewith, means for supporting said armature for vibration, magnetic pole pieces on opposite sides of opposite ends of said armature, a mechanical load connected to the free end of said lever arm, a vibrating spring, said spring having its vibratile end secured to a point on said lever arm, said spring being longi tudinally adjustable with reference to its support to control the natural frequency of the vibrating system.

23. In apparatus of the class described, an armature assembly comprising, in combination, a laminated armature of magnetic material, a lever arm of channel cross-section having wings embracing opposite ends of said armature and rigidly secured thereto, and a torsion spring extending transversely through said armature and having its opposite ends projecting therefrom.

24. In apparatus of the class described, an armature assembly comprising, in combination, a

laminated armature of magnetic material, a lever arm of channel cross-section having wings embracing opposite ends of said armature, reinforcing plates positioned over said wings and extending over a portion of the sides of said channel member, rivets passing transversely through said reinforcing plates, wings and armature for securing said parts rigidly together, and additional rivets securing said reinforcing plates to the sides of said lever arm.

26. In apparatus of the class described, an armature assembly comprising, in combination, a lever arm of channel cross-section having wings embracing opposite ends of said armature, reinforcing plates positioned over said wings and extending over a portion of the sides of said channel member, a torsion spring passing transversely through said reinforcing plates, wings and armature, transverse rivets passing through said plates, wings and arinature on opposite sides of said spring, and additional means for rigidly securing the outer ends of said reinforcing plate to the side of said channel.

2'7. In apparatus of the class described, an armature assembly comprising, in combination, a laminated armature of magnetic material, a lever arm of channel cross section having wings embracing opposite ends of said armature, and having openings formed therein whereby parasitic resonance in the flat surface portion of said arm is minimized, said lever arm being rigidly secured tosaid armature, and spring mounting means rigidly secured to said armature.

28. The method of reducing undesired vibrations of sound frequency in a work element connected in motion repeating relation to a vibrating armature system mounted for vibration at sound frequency about an axis and connected to said work element at a point off said axis, which comprises arranging the constants of said armature system so that the center of percussion of said system with respect to the point of attachment to said work element lies on the vibratory axis of said armature.

29. The method of reducing undesired vibrations in a work element connected to a vibratory armature system mounted for vibration about an axis'and connected to, said work element at a point ofi said axis, which comprises so distributing the mass of said vibrating system that forces applied thereto at the axis thereof produce substantially no translatory movement at the point of attachment of said work element.

30. In an electro-mechanical translating device of the class described, in combination, an armature'mounted for vibration in a magnetic field, magnetic pole pieces on opposite sides of opposite ends of said armature, a work element, and a lever arm mounted on said armature and embracing said armature on opposite sides thereof and rigidly secured thereto, and having a connection between its-free end and said work element.

31. In apparatus of the class described, in combination, a laminated armature of magnetic material, a rigid lever arm rigidly secured thereto, and a torsion spring extending transversely through the lamination'of said armature and projecting therefrom, said lever arm embracing said armature on opposite sides thereof and said spring projecting through said lever arm, the cross-sectional area of said spring within said armature being greater than the cross-sectional area of the flexing portion of said spring.

32. The method of reducing the efiects of undesired high frequency resonance in an electromagnetic translating device having an armature system spring biased to neutral position, and connected to a load through a lever arm, which comprises utilizing a lever arm of such high step-up ratio that the resistance of said armature spring to transverse deflection may be neglected, and so distributing the mass of said armature system that transverse deflection of said spring applies substantially no translatory force to said load.

ANDREW R. BARFIEIJ). 

